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handlers: reject protocol-relative URLs in redirect validation #5281

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handlers: reject protocol-relative URLs in redirect validation #5281

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6fdd829
handlers: reject protocol-relative URLs in redirect validation
adilburaksen May 16, 2026
b188f3e
Reject backslash and control/whitespace bypasses in redirect validation
adilburaksen Jun 11, 2026
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11 changes: 10 additions & 1 deletion src/appengine/handlers/base_handler.py
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Original file line number Diff line number Diff line change
Expand Up @@ -45,7 +45,7 @@
# https://github.com/google/closure-library/blob/
# 3037e09cc471bfe99cb8f0ee22d9366583a20c28/closure/goog/html/safeurl.js
_SAFE_URL_PATTERN = re.compile(
r'^(?:(?:https?|mailto|ftp):|[^:/?#]*(?:[/?#]|$))', flags=re.IGNORECASE)
r'^(?:(?:https?|mailto|ftp):|(?!//)[^:/?#]*(?:[/?#]|$))', flags=re.IGNORECASE)


def add_jinja2_filter(name, fn):
Expand Down Expand Up @@ -130,6 +130,15 @@ def make_logout_url(dest_url):

def check_redirect_url(url):
"""Check redirect URL is safe."""
# Browsers strip ASCII tab/newline and leading control/whitespace characters
# from URLs and normalize backslashes to forward slashes, so inputs such as
# `/\\evil.com`, ` //evil.com` or `\r\n//evil.com` would otherwise pass the
# scheme-relative check below and then navigate to an external host. Reject
# any input with a backslash, a control character, or leading/trailing
# whitespace before validating.
if (not url or url != url.strip() or '\\' in url
or any(ord(char) < 0x20 for char in url)):
raise helpers.EarlyExitError('Invalid redirect.', 403)
if not _SAFE_URL_PATTERN.match(url):
raise helpers.EarlyExitError('Invalid redirect.', 403)

Expand Down